Electrically conducting sealing compound for spark plugs

Details Electrically conducting sealing compound for spark plugs Electrically conducting sealing compound for spark plugs

1994-09-30

1996-10-15

O'Shea, Sandra L.

Electric lamp and discharge devices

Spark plugs

With particular connector structure

313136, 313144, H01T 112

Patent

active

055657303

DESCRIPTION:

BRIEF SUMMARY
PRIOR ART

The invention relates to an electrically conducting sealing compound for a spark-plug contact core which is disposed between a terminal-side section and a spark-side section of a centre sparkplug electrode and which essentially contains a fusible glass component and a powdered, electrically conducting component. Sealing compounds are used for the leakproof sealing of the centre electrode in the insulator body of the spark plug and for radio interference suppression. The seal is also referred to as a resistance seal. In this connection, the resistance seal comprises a contact core with good electrical conduction on the terminal-side section and on the spark-side section, respectively, of the centre electrode and a resistance core situated in between.
German Patent Specification 22 45 403 discloses a generic contact-core sealing compound containing borosilicate glass as the fusible component and graphite and soot as the electrically conducting component. In addition, it is pointed out that sealing compounds containing only graphite as the conductive component are unsuitable because of the high proportions by volume of graphite required and the difficulties resulting therefrom during the processing and because of the inadequate gas impermeability.
It was observed that spark plugs having a known composition of the sealing compound of the contact core undergo failure under high thermal load because the soot or carbon black contained in the contact core on the central electrode burns out. The replacement of the carbon black by metal powder also proves unsatisfactory because of inadequate stability under high electrical load.
The resistance core is composed of glass and, as fillers, other inorganic additives which do not melt at the sealing temperatures applied, and also of carbon as the electrically conductive component. In this connection, the carbon is added as soot or carbon black and/or by means of an organic binder which forms carbon. German Patent Specification 32 26 340, inter alia, discloses the use of carboxymethylcellulose (CMC) as an organic binder.


SUMMARY AND ADVANTAGES OF THE INVENTION

It was found that the proportion of graphite necessary to function as the sealing core and the contact core for resistance seals does not have to be set so high that the disadvantage mentioned in German Patent Specification 22 45 403 occurs. In contrast to this, the sealing compound according to the invention intended for the contact core and having an electrically conducting component which contains graphite which is, at least approximately, free of crystal structures of carbon black, has the advantage that the sealing compound used to produce the contact core can be processed appreciably better than with the finely divided soot which tends to agglomerate.
The measures disclosed in the specification and specified in the subclaims make advantageous further developments of the basic resistance seal according to the invention possible. Because of the higher tendency to oxidation of fine-grained graphite, it is particularly advantageous if the graphite has as low as possible a grain-size component below 5 .mu.m. The grain size is limited in the upward direction by the number of contact points between the graphite grains, said number becoming ever smaller with increasing grain size. Particularly expedient is as narrow a grain-size distribution as possible, with a mean grain size between approximately 20 and 50 .mu.m, preferably from 30 to 40 .mu.m, while the proportion of the grain size below 10 .mu.m should be lower than 5% and of the grain size over 96 .mu.m should be below 10%. To reduce the oxidation of the graphite, the contact core may contain up to 4% by volume of fine-grained aluminium powder as a reducing agent. A particularly good stability under high electrical load is achieved if, after the sealing compound has been sealed, the contact core on the terminal-side section of the centre electrode is thicker than the contact core on the spark-side centre electrode.


BRIEF DESCRIPTION OF THE DRAWING


REFERENCES:
patent: 3909459 (1975-09-01), Friese
patent: 3931055 (1976-01-01), Friese
patent: 4514656 (1985-04-01), Damson et al.

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